Novel photographic process of diffusion transfer of x-ray image with phosphorescent luminophor

ABSTRACT

X-ray exposures may be reduced by using a film containing an integral X-ray intensifier screen composed of phosphorescent luminophors, and intensifying the latent image by actinic radiation emitted after the X-ray exposure has ceased. The thus exposed and intensified film is diffusion transfer processed to provide a silver transfer image.

United States Patent Joel M. Peisach Hudson, Mass. 623,990

Mar. 17, 1967 May 4, 197 l Polaroid Corporation Cambridge, Mass.

Inventor Appl. No. Filed Patented Assignee NOVEL PHOTOGRAPHIC PROCESS OF DIFFUSION TRANSFER OF X-RAY IMAGE WITH PHOSPHORESCENT LUMINOPHOR 6 Claims, 3 Drawing Figs.

US. Cl 250/65, 96/29 Int. Cl ..G0ln 21/34 Field of Search 96/29, 76, Y 82; 250/65 Primary Examiner-Norman G. Torchin Assistant ExaminerJohn L. Goodrow Attorneys-Brown and Mikulka and Mark C. Jacobs ABSTRACT: X-ray exposures may be reduced by using a film containing an integral X-ray intensifier screen composed of phosphorescent luminophors, and intensifying the latent image by actinic radiation emitted after the X-ray exposure has ceased. The thus exposed and intensified film is diffusion transfer processed to provide a silver transfer image.

I SUPPORT F I G. I

' SUPPORT RECEIVING LAYER MULSION LAYER INTENSIFIER SCREEN SUPPORT FIG. 2

PTURABLE CONTAINER CONTAINING PROCESSING COMPOSITION SPREADER SHEET MULSION LAYER CEIVING LAYER TENSIFIER SCREEN SUPPORT. I

KNEESQ FIG.3

I NVENTOR. Mod 7720. Plaza.

3W 3? 7721M 7%wh 25. /acoia ATTORNEYS UPTURABLE CONTAINER CONTAINING PROCESSING COMPOSITION- NOVEL PHOTOGRAPHIC PROCESS or DIFFUSION TRANSFER or X-RAY IMAGE wrra PHOSPHORESCENT LUMINOPHOR transfer techniques.

Other objects of the invention will in part be obvious and 4 will in'part appear hereinafter.

The invention accordingly comprises the products possessing the features, properties and the relation of components, and the processes involving the several steps and'the relation and order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

Fora fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:

In the embodiment illustrated by FIG. 1, a support bears on its surfaceflexible phosphor layer 14, which in turn bears on its surface emulsion layer 13. Layer 13 may optionally be I overcoated with an antiabrasion layer 4, such as gelatin.

- for preparing X-ray images, as does U.S. Pat. No. 3,185,841,

also issued to Edwin H. Land, on May 25, 1965. U.S. Pat. No. 2,887,379, issued to Blake et a]. on May 19, 1959, discloses a film structure containing an intensifier screen, which may be utilized in diffusion transfer processes for obtaining X-ray' images.

The present invention relates to improvements over the above cited patents, and more specifically to novel radiographic film structures wherein a phosphorescent material is utilized as the luminophor of an intensifier screen in place of at least part of the luminescent material utilized as the luminophor in the intensifier screens of the priorart. It is to be understood that a phosphorescent luminophor can be utilized in those diffusion transfer processes disclosed in the Land patents, supra, as well as in diffusion transfer processes of the type disclosed by Blake et al. as well as other diffusion transfer processes known in the art.

The use of intensifier screens in radiography is well known. As is understood in the art, such screens are employed primarily to reduce the amount of exposure to X-ray or other radioactive waves necessary to form a latent image. Use of such screens also gives a more desirable,i.e., longer, contrast range curve. The advantages to be obtained by subjecting liv-' ing organisms to lesser amounts of radiation are well known to those familiar with the radiographic art. Accordingly, it has become quite common to employ such screens in radiography.

The generic term luminophor is subclassifred into two categories; namely, fluorophors, i.e., fluorescent materials, and phosphors, i.e., phosphorescent materials; A luminophor has the property of being able to convert a part of the energy absorbed from activation by exposure to particular wavelengths of the spectrum, e.g., X-rays, into emitted radiation of theexcitation of the luminophor. A phosphorescent material is said to possess an afterglow, i.e., continuance of emission after cessation of excitation, longer than about seconds for optical photons, whereas for fluorescent materials the afterglow is less than 10' seconds. The basis for the demarcation at l0 seconds between fluorescence and phosphorescence is explained in detail in An Introduction to Luminescence of Solids" by Humboldt W. Leverenz, published by John Wiley and Sons, me, 1950, at page 149, et seq.

The intensifier screens of the prior art have always used a fluorescent material as the luminophor.

In operation, the X-ray package is exposed to X-ray radia- 5 tion. The silver halide stratum is substantially insusceptible to X-ray, i.e., it possesses little stopping power for this type of radiation. Thus, it permits the radiation to penetrate through it to the luminophor of the intensifier screen. The luminophor, upon absorbing X-ray radiation, will emit quanta in a particular segment of the electromagnetic spectrum. The photosensitive stratum is then exposed by the quanta emitted from the luminophor. Usually these quanta are in the form of visible light.

The amount of quanta received by the photosensitive element is proportional to the degree of X-ray excitation of the luminophor at a given level of X-ray tube voltage. Thus, it is seen that latent image formation is derived from bombardment with two sources of quanta; namely, directly by the X- rays, and indirectly from the visible light emitted by the luminophor. lt therefore follows that the use of an intensifier screen results in a lowering of the exposure time and/or the amount of radiation necessary to form the latent image.

The utilization of such an intensifier screen in accordance with the present invention may best be understood byreferring to the drawing.

FIG. 1 illustrates one embodiment of this invention wherein the several layers are coated upon each other. HO. 2 illustrates another embodiment 'of the invention as shown therein. A rupturable container of processing'solution, such as that disclosed in U.S. Pat. No. 2,565,378, is provided between the emulsion layer 13 and the receiving layer 12. A suitable support 15, such as those of a nature to be described below, is provided for both the receiving layer 12 and the intensifier screen 14, i.e., the layer containing the phosphorescent luminophoL'The radiographic product illustrated in H6. 2 is meant to be utilized in a diffusion transfer process similar to those previously described. Subsequent to exposure, the rupturable container 16 is broken, for instance, by passage through a pair of rollers, and the processing composition is thereby spread in a uniform layer between the emulsion layer containing the latent image and the receiving layer.

FIG. 3 illustrates still another embodiment of the invention as shown therein. Arupturable container of processing composition 16 is provided between a spreader sheet which is a sheet of suitable material such as cellulose acetate, having a planar surfaceon the innerface, and being of sufficient rigidity to confine the processing composition within the gap between itself and the emulsion layer 13. The emulsion layer 13 is superposed upon the intensifier screen 14, which is coated upon a support 15.

In the embodiment of FIG. 3, imbibition is to occur through a permeable emulsion layer to the image-receiving layer in a manner as has been described previously in the prior art.

As will beapparent to those skilled in the art, a positive image is obtained by the transfer by imbibition, of an imagewise distribution of soluble silver complex formed from unexposed silver halide within the emulsion layer. The positive image may be revealed for viewing by stripping the imagereceiving layer from the emulsion layer.

Support 15 is either opaque to visible radiation and transparent to X-rays and other radioactive rays, or, comprises a transparent base material, preferably a plastic, for example, a cellulosic ester such as cellulose acetate, or a synthetic polymer such as polyvinyl chloride, polyvinyl acetate, polystyrene, and polyethylene terephthalate, etc. Where the support is transparent, it should be apparent that it must be provided with a light-opaque backing, such as black paper, or the film unit must otherwise be encased in a light-opaque material to'prevent exposure to visible light.

The intensifier screen 14 of the presentinvention may comprise any phosphorescent luminophor known in the art. Particularly satisfactory results may be obtained utilizing the following phosphorescent materials in the intensifier screen:

2. ZnMgF Mn 3. zno and ZnCdS Cu 4. MgF Mn 5. ZnS Cu While ZnS, as well as other compounds, are commonly known in the art as luminophors for X-ray intensifier screens,

to the best of the inventors knowledge no one has used a phosphorescent form of such compounds in the intensifier screens of the prior art to intensify a latent image. This is believed to be the case because of the preconceived notion of the necessity to quench a phosphorescent material, as well as the associated problem of ghost images formed by movement of the screen during the period of continued emission subsequent to cessation of exposure if quenching is not effected.

Whereas the technical details relating to the preparation of phosphorescent luminophors are unknown to the inventor, the fact remains that suppliers of such materials are able to prepare luminophors which possess the property of phosphorescence and such compounds are commercially available in the market place.

It has been found that satisfactory X-ray images can be obtained when one utilizes an intensifier screen wherein the phosphorescent luminophor is coated upon the support within the range of 5 to 100 mg. of phosphor per square centimeter of base.

' If the intensifier screen 14 is sufficiently strong and also resilient, it is seen that it may be self-supporting and that a separate support material for this layer can be eliminated. If such is the case, a suitable means such as a light opaquing layer, should be provided to shield the film from actinic radiation.

The image-receiving layer 12 may be any of those heretofore used in photographic diffusion transfer processes. In a preferred embodiment, image-receiving layer 12 comprises an image-receiving layer containing'deacetylated chitin, which is described and claimed in the U.S. Pat. No. 3,087,815, issued to Ryan et al.

The emulsion layer 13 may be any suitable photosensitive emulsion known in the art. 1

Where desired. such as in the embodiment of FIG. 3, a stripping layer may be positioned between'the photosensitive emulsion layer '13 and the image-receiving layer 12 to facilitate separation of these layers. Materials suitable for use as stripping layers are well known in the art.

The film structure described above is employable in a diffusion transfer process similar to that described in the aforementioned U.S. Pat. No. 2,565,378, to obtain a positive radiographic image.

At some time subsequent to exposure, emulsion layer 13 containing the latent image is contacted with a processing composition 1 1, preferably as a relatively viscous layer.

. The processing composition may be any. of those known in the art, e.g., a film-forming processing composition such as those disclosed in U.S. Pat. Nos. 2,543,18l and 2,565,378. It may comprise, for example, a developing agent such as hydroquinone, an alkali such as sodium hydroxide, a silver halide complexing agent such as sodium thiosulfate, and a high molecular weight film-forming thickening agent such as hydroxyethyl cellulose or sodium carboxymethyl cellulose. Various other additives such as accelerating developing agents, preservatives, antifogging agents, and the like may also be included. All of these materials are preferably in aqueous solution. These photographic agents are preferably contained in solution in the processing liquid prior to the spreading thereof, but they may be in part or wholly dissolved into the processing composition as it is spread upon emulsion layer 13.

The phosphorescent luminophor is not quenched by the processing composition but rather one of the ingredients of the processing composition complexes the unexposed silver halide and starts the imbibition process. The developing agent in the processing composition reduces the exposed silver halide to free silver. The silver halide which was not exposed from the radiation emitted from the screen is now complexed and becomes incapable of responding to the light from the screen. Thus, while the screen may continue to phosphoresce, the effect is nullified such that the effect is the same as if quenching had taken place.

' allowing the phosphorescent luminophor to continue its afterglow, i.e., continue emission of visible light after cessation of excitation, it is seen that this additional quanta of light from the intensifier screen will actually intensify the latent image. Such'a phenomenon is known as latensification. Insofar as the X-ray exposure is concerned, the highest possible voltages may then be utilized in this invention since the actual exposure time may be of a shorter duration than that commonly required. The use of the higher voltages insures minimal absorption of ionizing radiation by the subject. This is true because the higher the voltage the less the absorption due to the greater proportion of har X-rays which are the deep penetrating shortwave length X-rays. These rays are least absorbed by the human body. The danger that arises from the soft X-rays is the fact that upon absorption by the body, they give rise to an ionizing effect upon certain constituents of the body chemistry. It must be realized that only in an integral X- ray system can this substitution of a phosphorescent for a fluorescent material be utilized, for the reasons set forth to follow.

In the X-ray system of the present invention, it is not necessary to quench the phosphor prior to developing the image. The failure to quench a phosphorescent luminophor when such is used in a nonintegral X-ray package would result in exposure in the previously unexposed areas should one attempt to move the film while phosphorescence is still occurring. In the embodiments of the present invention the emulsion is either directly laminated or bound to the intensifier screen such as in the embodiment of FIG. 2, such that there is a continuous pointto-point relationship between said emulsion and said intensifier screen; or as in the embodiment of FIG. 3, the emulsion is bound tothe receiving layer which is bound to intensifier screen such that there is a continuous point-to-pointto-point relationship. The positioning of the intensifier screen in relation to the emulsion and to a degree the receiving layer is such that the problem of ghost images which could arise in the prior art from the utilization of a phosphorescent intensifier screen cannot arise in the processes of the present invention. Thus it is seen that movement of the intensifier screen, since it is done in conjunction with the movement of the emulsion, will have no detrimental effect upon the image.

The development of the embodiment of FIG. 1 is similar to those described in the art, wherein diffusion takes place from the emulsion through a permeable antiabrasion layer to a receiving layer.

When the embodiment set forth in FIG. 2 is processed through a roller or other spread system in conformance with the well-known diffusion transfer processing techniques, the ingredients of the processing composition help to maintain an photosehsitiveelement during imbibition. After processing,

the image-receiving material is stripped from the integral emulsion-intensifier screen unit and the latter is discarded.

When the embodiment of H6. 3 is processed, the impermeable spreadersheet acts to restrain the processing composition and to help spread the composition uniformly. lmbibition occurs through the permeable emulsion layer.

' When the embodiment of FIG. 3 is processed, the highly alkaline 'nature of the processing composition may be used to weaken the adhesive capacity between the emulsion and the receiving layer to an amount less than the adhesive capacity of the in situ formed laminate between the spreader sheet and the emulsion such that the emulsion can be stripped away from the receiving layer when the spreader sheet is separated.

An additional advantage to be gained when the intensifier.

screen is also activated by ultraviolet radiation to give rise to the emission of visible light, is the fact that whether the support material be transparent or opaque, the resulting reflection print may be viewed under ultraviolet light as if it were a transparency, as per procedures described and claimed in copending U.S. application Ser. No. 410,045, filed Nov. 3-, 1964.

When one views such a reflection print under ultraviolet light, there is obtained a brilliance and contrast usually associated only with a transparency.

The following example is intended to show by illustration only, and not in a limiting sense, the novel film structure of this invention.

EXAMPLEl A base prepared from baryta paper overcoated with TiO dispersed in polyethylene was coated with a layer of phosphorescent zinc-cadmium sulfide, activated with silver, dispersed in polyvinyl butyral. The coating was of a coverage of 20 milligrams of luminophor per square centimeter of base.

EXAMPLE ll A sheet of the photographic film prepared as in Example l was given a 30 second unscreened X-ray exposure at 68 k.v.p.,

100 MA. 48-inch'distance through an aluminum step wedge. A positive transfer image was then obtained, in accordance with conventional diffusion transfer techniques, by imbibing the X-rayexposed film for 30 seconds with a developing composition similar to the one used in Polaroid Ty e 4.7 Land film. The positive image was revealed by stripping the silver receiving layer from the silver halide emulsion layer.

For convenience, the expression X-ray has been used in the specification and in the claims and is intended to cover all photographically useful radioactive rays such as those emanating from an X-ray tube, radium, or radioactive isotopes.

Since certain changes may be made in the above product and process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

lclaim:

1. ln a diffusion transfer photographic process for forming an X-ray image wherein a photosensitive element comprising a support carrying an intensifier screen and a photosensitive silver halide emulsion is exposed to X-ray radiation and the exposed photosensitive element is processed to form a diffusion transfer silver image on a silver receptive layer in superposed relationship with said silver halide emulsion, said support being transparent to said X-ray radiation and said intensifier screen being positioned between said support and said silver halide emulsion, the improvement whereby said X-ray exposure is reduced by using an intensifier screen comprising a phosphorescent luminophor which emits light actinic to said silver halide upon exposure to X-ray radiation and continues to emit said actinic radiation after said X-ray exposure ceases, and including the step of delaying said diffusion transfer processing for a sufficient period of time after said X-ray exposure has ceased to permit the continued emission of said actinic radiation to intensify the latent image formed during said X-ray exposure.

2. A diffusion transfer photographic process as defined in claim 1 wherein said silver receptive layer is coated on said support between said intensifier screen and said silver halide emulsion, and said silver halide emulsion is removed after said transfer image has been formed.

3. A diffusion transfer photographic process as defined in claim 1 wherein said phosphorescent luminophor is zinc cadmium sulfide activated with silver.

4. A diffusion transfer photographic process as defined in claim 1 wherein said phosphorescent phosphor is zinc sulfide activated with copper.

5. A diffusion transfer photographic process as defined in claim 1 wherein said phosphorescent phosphor is zinc cadmium sulfide activated with copper.

6. A diffusion transfer process as defined in claim 1 wherein said phosphorescent phosphor is coated on said support at a coverage of 5 to mg. per square centimeter. 

2. A diffusion transfer photographic process as defined in claim 1 wherein said silver receptive layer is coated on said support between said intensifier screen and said silver halide emulsion, and said silver halide emulsion is removed after said transfer image has been formed.
 3. A diffusion transfer photographic process as defined in claim 1 wherein said phosphorescent luminophor is zinc cadmium sulfide activated with silver.
 4. A diffusion transfer photographic process as defined in claim 1 wherein said phosphorescent phosphor is zinc sulfide activated with copper.
 5. A diffusion transfer photographic process as defined in claim 1 wherein said phosphorescent phosphor is zinc cadmium sulfide activated with copper.
 6. A diffusion transfer process as defined in claim 1 wherein said phosphorescent phosphor is coated on said support at a coverage of 5 to 100 mg. per square centimeter. 